The youtube video I linked to shows them using the brace miter box, and the braces. They can be a bit longer or shorter, to fit your frame. I don't think exact lengths are important here.
Typical practice in old barn frames around here (and anywhere else I have seen) is that diagonal brace tenons are not rectangular in profile, but sort of tapering. The face by the inside angle of the joint follows straight along the grain, and the face by the outside angle cuts back at 45 degrees, fitting the mortise. The mortise has the upper edge at 45 degrees to the post and the bottom perpendicular to the post (horizontal), and a short flat bottom parallel to the grain of the post. This makes the tenon strong with no angled corners that could break off, and wide
enough to take the peg hole safely. Note - the video shows similar tenons in the first few seconds.